1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, or a light-emitting device each including an oxide semiconductor.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics. An electro-optical device, an image display device, a semiconductor circuit, and an electronic device each include a semiconductor device in some cases.
2. Description of the Related Art
A technique in which a transistor is formed using a semiconductor layer has attracted attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (also simply referred to as a display device). As semiconductor materials applicable to the transistor, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
It is known that in a transistor in which a channel is formed in an oxide semiconductor, part of hydrogen contained in the oxide semiconductor forms a donor state to increase carrier density. Thus, to obtain stable electrical characteristics of a transistor using an oxide semiconductor, some measures need to be taken to reduce the concentration of hydrogen contained in the oxide semiconductor. For example, Patent Document 1 discloses a method for reducing the concentration of hydrogen that might be contained in an oxide semiconductor layer. In the method, the oxide semiconductor layer and an insulating layer in contact with the oxide semiconductor layer are formed in a film formation chamber in which the concentration of impurities such as hydrogen is reduced. Patent Document 2 discloses a method for preventing entry of water, hydrogen, hydrocarbon, or the like into an oxide semiconductor layer. In the method, a resist mask used in processing for forming the oxide semiconductor layer is removed by ashing instead of chemical liquid treatment.